Are you happy? This question is asked of people by friends, parents and psychiatrists alike. What happiness consists of for each person seems, at first glance, to be entirely subjective in that is it up to each individual person to define what the happy-making ingredients of her life are. This dissertation centrally involves an interpretation of Aristotle’s eudaimonia, often translated as ‘happiness’. Aristotle’s Nicomachean Ethics is an inquiry into the chief good for human beings, and according to Aristotle everyone agrees that this chief good is ‘happiness’, however there is major disagreement about what ‘happiness’ consists of. What follows critically interprets Aristotle’s eudaimonia through a close reading of his arguments. Once Aristotle’s eudaimonia is explicated, it is used to question the supposedly subjective conception of happiness that the happiness literature argues is pervasive. Finally, Aristotle’s eudaimonia is defended as a theory of well-being against a charge of perfectionism. It is argued that Aristotle’s eudaimonia commits its adherents to maximising virtuous activity at all times, that is, to perfect themselves. It is this interpretation of Aristotle that seeks to undermine eudaimonia as a plausible theory of well-being, and I end this dissertation by providing a response to the objection from perfectionism. This project attempts, fundamentally, to show that Aristotle’s eudaimonia is not simply an intellectual curiosity: studying eudaimonia can help change the way we live our lives, and for the better